Broadband microwave window



April 12, 1960 J. E. BURR, JR BROADBAND MICROWAVE wmnow 2 Sheets-Sheet 1 Filed Dec. 2. 1958 INVENTOR.

JOHN E. BURR JR. BY g a ATTORNEY April 12, 1960 J. E. BURR, JR

BROADBAND MICROWAVE WINDOW 2 Sheets-Sheet 2 Filed Dec. 2, 1958 FIG. 5

INVENTOR. JOHN E.BURR JR. BY a fifi/ z" ATTORNEY United States Patent O 2,932,806 BROADBAND MICROWAVE wmnow John ErBurr, Jr., Wenham,

Laboratories Inc., Beverly, Massachusetts Mass assignor to Bomac Mass, a corporation of The present invention relatesto waveguide transmission apparatus and more particularly to an: improved broadband non-resonant window for such apparatus" and a novel method manufacture.

Prior art windows for use in waveguide transmission structures particularly at microwave frequencies generally comprise a metallic apertured frame with a dielectric enclosure-sealed across the aperture. A metal alloycomprising iron, cobalt and nickel has proven extremely useful for glass-to-metal seals in. view of a comparative thermal coeffici'ent expansion between the two materials. The alloy referred to by the trademark Kovar has been widely adopted in fabricatingsuchseals. The windows: of the type to be described in the invention are widely used in microwave transmission lines to pressurize one section while an adjacent section is at atmospheric pressure. Such pressurization increase the power handling characteristics of the line by preventing arc-over and spitting at the high power levels. Furthermore, non-resonant windows are employed to provide a gas-retaining chamber where desired, as well as to prevent introduction of dust or humidity in the transmission path. It is desirable that such window structures cover a wide frequency band with a low voltage standing wave ratio and low insertion loss characteristic.

Recent developments in the art have evolved the use of mica window members sandwiched between metallic frame members for broadband transmission systems. An inherent disadvantage exists, however, in that mica splits very easily along the edges and the resultant windows will prove unsatisfactory for pressurization applications.

Accordingly, it is an object of the present invention to provide an improved broadband non-resonant waveguide window for microwave transmission systems.

It is a further object of the invention to provide an improved non-resonant window for waveguide transmission systems that is easy and less costly to manufacture.

Other objects, features and advantages will be evident after consideration of the following detailed description and the accompanying drawnigs, in which:

Figure l is an exploded perspective view of the illustrative embodiment;

Figure 2 is a bodiment;

Figure 3 is a partial enlarged cross-sectional view of the embodiment along the line 33 in Figure 2;

Figure 4 is a perspective view of an alternative assembly partially broken away to reveal the dielectric member; and

Figure 5 is a partial enlarged detailed cross-sectional view along the line 5-5 in Figure 4.

Referring now to the drawings, the embodiment shown in Figure 1 is illustrative of a window configuration to be employed in the so-called ridge waveguide to transmit microwave frequencies of very wide band width. While this specific embodiment has been selected it will be realized that the invention will be equally applicable to all other transmission hollow-pipe line configurations perspective view of the assembled emthe region between the 2,932,806 Patented Apr. 12, 1960 such as rectangular or circular. A front window frame 1 with suitable mounting holes 2 is provided with a substantially H-shaped aperture 3 having two reentrant central portions 4 and 5. The aperture dimensions will approximate the cross-sectional area of the inside of the hollow-pipe waveguide line and is, therefore, a nonresonant structure. A similar frame 6 with aperture 7 and reentrant portions 8 and 9 will serve as the back member of the assembly. In accordance with the teachings of the invention window frames 1 and 6 will no longer be required to be fabricated from the alloy Kovar since a direct glass-to-metal seal has been eliminated. Instead, any desirable metal may be used including light weight aluminum which has a distinct advantage in air borne equipment.

The enclosure material in the embodiment comprises a dielectric material of thermoplastic resin composition namely polytetrafluoroethylene which is commercially available under the trademark Teflon. A sheet of this material having a bendable coating 12 on both planar surfaces prepared by the manufacturer by a surface etching process forms the window member 10. The window has the same outer dimensions as frame members 1 and moved in the area 13 corresponding to the cross-sectional area of the inside of the selected waveguide line.

Surrounding the area 13, I provide a metallic interlacing in the form of wire stitching 14 extending through the window member 10 and following the same outline 7. The stitching is located in edge of area 13 and the outer peripheral edge of the window member 10. It is desirable that good electrical continuity be provided between metallic frames 1 and 6 and, also the effect of the stitching on the overall voltage standing wave ratio measurement must be taken into consideration. After The complete window assembly is shown in Figure 2 and detailed in Figure 3. The overall assembly is formed simply by first coating the window member with a suitable epoxy resin to form layer 15 over all the areas surrounding the exposed area 13 including the stitcr or providing gas tight enclosures.

An alternative embodiment of the invention is shown in Figures 4 and 5 and comprises in this instance two metallic frames 16 and 17 having rectangular apertures 18 illustrated for the sake of clarity. The Window member 19 in this embodiment is selected from a ceramic material with a conductive metallic coating 20 over both planar surfaces with the exception of the area 21 which corresponds to the apertures 18. A plurality of holes 22 are provided surrounding the area 21 prior to the application of the coating20. As a result the coating will as at 23 to provide a metallic interlacing similar to the stitching in the previous embodiment. The window member 19' will now be sandwiched between the frames 16 and 17 and brazed together by the common ceramic-to-metal techniques including the m0lybdenummanganese. The brazed metallic layer is indicated at 24 and unites the assembly into a composite unit.

The window assembly of the invention will be readily adaptable to all waveguide type lines or other apparatus and excellent results have been attained with this nonresonant type structure. Minimum voltage standing wave ratios below those measured with conventional glass or mica prior art structures have been achieved together with accompanying low insertion loss measurements. Additionally, the metallic interlacing assures excellent electrical continuity heretofore attained only with metallic gaskets in glass structures and unattainable in mica structures wherein the mica is embedded in a glass frit mixture between the metallic frames.

Alternate embodiments, as well as modifications and variations will be readily apparent to artisans skilled in the art. It is my intention to cover such alterations, modifications and variations as fall within the spirit and scope of the invention as defined in the appended claims.

What I claim is:

l. A broadband non-resonant window assembly for hollow-pipe waveguide transmission apparatus comprising a window member of a thermoplastic resin material having a bendable coating on all planar surfaces with the exception of an exposed area equivalent to the inside wall dimensions of the hollow-pipe waveguide, a metallic interlacing disposed intermediate the edge of said exposed area and the peripheral edges of said window member and having substantially the same configuration as the hollow-pipe waveguide, a metallic apertured frame member secured to each planar surface of said window member by a bonding layer of an epoxy resin material to define a composite member adapted for mounting in suitable waveguide structure with said thermoplastic resin material enclosing all of the inside cross-sectional areamf said waveguide.

2. A broadband non-resonant window assembly for use in pressurized hollow-pipe waveguide transmission systems to seal separable sections, said window assembly comprising a member of a thermoplastic resin material having a bendable coating on all planar surfaces with the exception of an exposed area equivalent to the inside wall dimensions of the hollow-pipe waveguide, a continuous conductive wireinterlacing surrounding the exposed area and having substantially the same configuration as the hollow-pipe waveguide, a pair of metallic frame members each defining a central aperture equivalent to the inside wall dimensions of the hollow-pipe waveguide, each of said frame members being secured to a planar surface of said thermoplastic resin member by a bonding layer of an epoxy resin material to define a composite member adapted for mounting between sections of hollow-pipe waveguideto provide a gas-tight joint with said thermoplastic resin member enclosing all of the inside cross-sectional area of said waveguide.

References Cited in the file of this patent UNITED STATES PATENTS 2,636,926 Wilson et al. Apr. 28, 1953 2,674,644 Goodloe Apr. 6, 1954 2,883,631 Blackadder et a1 Apr. 21, 1959 2,885,459 Pulsifer et a1 May 5, 1959 OTHER REFERENCES Yates et aL: RCA TN No. 189, Aug. 18, 1958. 

